The invention relates to the field of microelectronics. More specifically, it relates to monolithic integrated circuits which include inductive components such as those used especially for applications in radiofrequency telecommunications.
It also relates to a process for fabricating such components, which makes it possible to obtain compact circuits having electrical characteristics, and especially a Q-factor, which are superior to those of existing components.
As is known, integrated circuits are being used more and more in microwave and radiofrequency techniques.
In these applications, it is important to be able to use tuned oscillating circuits consisting of a capacitor-inductor combination.
Now, such circuits must be produced so as to occupy smaller and smaller volumes. Furthermore, they must operate at higher and higher frequencies. Consequently, the electrical consumption of such components becomes a critical parameter, for example in cellular portable telephones, since the consumption has a direct influence on the autonomy of these appliances.
Thus, it is required that the passive components constituting the filters used in radiofrequency systems, and especially the inductors, occupy as small an area as possible within the integrated circuits, have as high an inductance as possible and as low an electrical consumption as possible.
Furthermore, it is known that the inductors incorporated into integrated circuits made of semiconductor material are exposed to the influence of parasitic capacitances formed by the various localized substrate regions near the inductors.
Thus, in practice, an inductor has an equivalent circuit in which various parasitic components are added to this inductance proper, these parasitic components causing this inductor to depart from its ideal performance.
Thus, a real inductor has a resistance corresponding to that of the metal of which it is composed.
Furthermore, the electrical behavior of the inductor is disturbed by parasitic capacitances which result from various layers, located above the substrate, of materials of poor electrical permittivity.
Furthermore, added to the parasitic capacitances of these various layers are a capacitance and a parasitic resistance corresponding to the influence of the semiconductor substrate located above the ground plane.
Furthermore, a parasitic capacitance exists between the various turns making up the inductor.
In document EP-0,969,509, the Applicant has described a solution making it possible to produce such inductors on a semiconductor substrate, by adopting an arrangement allowing the value of the parasitic capacitance existing between turns to be greatly reduced. Such a solution consists in producing the inductor by means of a metal strip deposited on the substrate, in etching said substrate in order to make a cavity beneath the strip forming the inductor and thus in suspending and distancing the inductor from the substrate.
By virtue of these arrangements, it is possible to use inductors at higher frequencies while still maintaining satisfactory behavior. It will be recalled that the optimum operating frequency is determined as being that at which the Q-factor is a maximum. The Q-factor is determined in a known way by the ratio of the imaginary part, or reactance, of the input impedance of an inductor to the real part thereof.
The solution described in the aforementioned document, although satisfactory, does not make it possible to significantly improve the Q-factor in the low-frequency ranges, that is to say ranges below half the optimum frequency which, in the typical applications of the invention, is close to a few gigahertz.
This is because, in this frequency range, the behavior of the inductor is strongly dependent on the value of the equivalent resistance, which corresponds to the electrical resistance of the metal strip making up the actual inductor.
Now, all inductors produced in integrated circuits are at the present time made of aluminum and small in size, especially having a very small thickness, typically less than 4 microns, thereby resulting in a high electrical resistance.
Thus, one of the problems that the invention aims to solve is that of the undesirable influence of the overall resistance of the winding forming the inductor, while still maintaining advantageous electrical characteristics, especially in terms of parasitic capacitance.
Many documents, such as especially documents U.S. Pat. No. 5,874,883, EP 0,782,190 or U.S. Pat. No. 5,834,825, describe integrated circuits which include on their surface inductors consisting of a metal strip. These devices have the aforementioned advantages relating to a low Q-factor when the metal strip is of conventional thickness, of the order of a few microns. Furthermore, these inductors are produced during the process for fabricating the actual integrated circuit, thereby increasing the technological constraints since their incorporation must be taken into account in the steps of the process. Finally, and above all, such inductors occupy a certain area on the semiconductor substrate. This area used by the inductor therefore cannot be used for implanting active regions in the semiconductor, which in turn reduces the useful density of the latter.
In document U.S. Pat. No. 5,478,773, it has been proposed to produce an inductor on a substrate by forming a copper strip by etching a copper layer. Unfortunately, in order to form the turns of the inductor it is necessary to firstly deposit a growth layer of sputtered copper and then to deposit a second layer of electrolytic copper. Next, a differential etching operation is carried out which preferentially etches the sputtered copper compared with the electrolytic copper. This differential etching is necessary in order not to damage the electrolytic copper parts which form the turns. Such operating precautions complicate the process and do not allow turns which are of a sufficient size to markedly improve the Q-factor to be obtained.
The invention therefore relates to a monolithic integrated circuit incorporating an inductive component and comprising:
a semiconductor substrate layer;
a passivation layer covering the substrate layer;
metal contact pads connected to the substrate and passing through the passivation layer in order to be flush with the upper face of the layer.
This integrated circuit is distinguished in that it also includes a spiral winding which forms an inductor and lies in a plane parallel to the upper face of the passivation layer, said winding consisting of copper turns having a thickness of greater than 10 microns, the winding ends forming extensions extending below the plane of the winding and being connected to the contact pads.
In other words, the inductor is mounted directly on the integrated circuit above the passivation layer. It is formed just after the process for producing the integrated circuit itself. It is therefore possible to create it on wafers from a very wide variety of sources, since its production is independent of the operations for fabricating the actual integrated circuit.
Such an inductor lies above the integrated circuit and not on the integrated circuit, so that the area of the integrated circuit located vertically below the inductor may include active regions in addition to the contact pads. The density of functions on the integrated circuit is therefore not decreased by the presence of the inductor.
The use of copper for producing the winding makes it possible to greatly reduce the equivalent resistance of the inductor. This reduction is made even greater by using turns having a thickness substantially greater than that of the metal strips currently used.
Consequently, the equivalent resistance is very greatly reduced, typically by a factor of ten, compared with the resistance of inductors produced at the present time in integrated circuits.
It follows that the Q-factor is very significantly greater than that of the inductors currently existing, especially at and above low frequencies.
Typically, the Q-factor of such coils is greater than the Q-factor of existing inductors by a factor of approximately ten.
In practice, the plane in which the inductor lies is advantageously away from the upper face of the passivation layer by a distance of more than 10 microns.
The reason for this is that it has been found to be important for the actual inductor to be sufficiently remote from the substrate to limit the phenomena of electrical losses within the substrate, which losses are observed at the operating frequencies of the circuits according to the invention. This distance must nevertheless not be too great, for fear of mechanically destabilizing the inductor.
Thus, for a distance greater than 10 microns, and preferably close to 30 microns, the electrical losses in the substrate are limited, while still ensuring good mechanical stability.
In a first embodiment of the invention, the component comprises a support layer made of benzocyclobutene on which the winding forming the inductor rests. Consequently, the inductor is mechanically stabilized on the support layer, thereby preventing the various turns of the inductor from vibrating against each other and ensuring good mechanical rigidity.
In a second embodiment of the invention, the support layer on which the winding forming the inductor rests is made of silica. According to two versions of the embodiment, the silica support layer is separated from the upper face of the passivation layer:
either by a layer of polyimide, or any other dielectric polymer;
or by a layer of air.
In the latter version of the embodiment, the electrical properties are optimized since the electrical permittivity of air is less than that of the polyimide.
According to another characteristic of the invention, the copper winding may be covered with a layer of gold or of a gold-based alloy, intended to passivate the copper and to prevent the copper oxidation phenomena which would degrade the electrical resistance characteristics, especially if the integrated circuit is used in wet, or even chemically aggressive, atmospheres.
According to another characteristic of the invention, the space between two consecutive turns of the winding is devoid of material, or more specifically filled with air, thereby greatly reducing the parasitic capacitance existing between each turn and therefore increasing the optimum operating frequency of the inductor.
As already stated, the invention also relates to a process for fabricating a monolithic integrated circuit incorporating an inductive component. Thus, starting with a semiconductor substrate covered with a passivation layer and comprising metal pads connected to the substrate, and passing through the passivation layer in order to be flush with the upper surface of said layer, the process according to the invention is distinguished in that it comprises the following steps in which:
a polyimide layer is deposited on the passivation layer;
a silica layer is deposited on said polyimide layer;
apertures are made in the silica and polyimide layers, said apertures emerging at the metal pads;
a metal growth sublayer is deposited on the assembly;
a layer of photosensitive resin is deposited on the metal growth sublayer;
the resin is exposed and the regions intended to form the lower face of the inductive component are removed;
a copper layer intended to form the strip of the inductive component is electrolytically deposited on the visible regions of the metal growth sublayer;
the rest of the photosensitive resin and the metal growth sublayer are removed.
In other words, the process according to the invention makes it possible to produce, directly on the integrated circuit, the inductors necessary for good operation of the circuit, after the process for fabricating the integrated circuit proper.
Consequently, the windings of the inductors are all produced simultaneously, during a series of steps constituting a continuation of the process for fabricating the integrated circuit itself. It is therefore unnecessary to make use of subsequent transfer operations which would consist in connecting inductors produced elsewhere on a finished integrated circuit.
According to one embodiment, the process includes an additional step of removing the polyimide layer, resulting in a component whose inductor is suspended above the passivation layer.
Of course, the invention also covers the process in which the polyimide layer is retained, especially in order to ensure stability of the inductor.
According to one embodiment, both the polyimide and silica layers are replaced by a single layer of benzocyclobutene, or any other equivalent material having a very low dielectric constant.
As already stated, the process may also include a step of passivating the copper strip by depositing a layer of gold or a gold-based alloy. Nevertheless, in a less perfected implementation of the process, provision may be made to cover the turns with a simple conventional passivation layer.
In practice, the process may advantageously include a step of depositing a metal forming a barrier layer on the metal pad. This eliminates the phenomena of copper migration into the aluminum, it being known that such phenomena can cause degradation of the active layer of the semiconductor substrate.
Advantageously, the process includes, after the step of depositing the barrier layer, a step of depositing a matching layer on the barrier metal layer. This step makes it possible to optimize the contact and promotes intermetallic adhesion, while limiting the parasitic capacitances appearing at the metal junctions.